This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
While 3G technologies such as HSPA and EV-DO based on CDMA introduced channel aware scheduling on the downlink in order to improve spectral efficiency, the CDMA nature of these technologies required fast power control on the uplink and hence channel aware scheduling was not possible. 3GPP LTE, which is based on OFDM, no longer requires fast power control on the uplink, and introduced a sounding reference signal (SRS) which makes channel aware scheduling on the uplink possible.
In order to fully exploit the channel aware scheduling capabilities of the LTE uplink, one must configure the system with the proper hybrid automatic repeat request (HARQ) operating point, which controls the rate at which retransmissions occur. The general philosophy at present is to target a low retransmission rate when the Doppler is low (i.e., the level of mobility is low) and target a higher retransmission rate when the Doppler is high (i.e., the level of mobility is high).
More generally, the setting of the HARQ operating point depends on the predictability of the channel state; that is, how accurate the base station scheduler's estimate of the mobile's channel state is when it decides to schedule the mobile compared to the actual channel state when the mobile finally transmits data. This is impacted by several factors such as the Doppler of the mobile (which depends on the mobile speed and carrier frequency), the period at which the SRS is configured to be transmitted, and the burstiness of the other cell interference.
Existing techniques to choose the proper HARQ operating point typically depend only on a speed or Doppler estimate. The Doppler is related to the mobile speed through the relationship:
      Δ    =          fv      c        ,where f is the carrier frequency in Hz, v is the velocity of the mobile in meters per second, and c is the speed of light in meters per second. By using the Doppler instead of the mobile speed, the carrier frequency is automatically taken into account. This technique relies on an accurate estimate of either the mobile speed or of the Doppler directly. In addition, the thresholds chosen in order to determine which HARQ operating point to use for a given mobile speed will be based on the rate at which channel state information is made available to the base station scheduler; in the LTE uplink, this is the period at which the sounding reference signal (SRS) is configured to be transmitted by the mobile. The 3GPP specification allows the SRS to be transmitted as fast as every 2 ms and as slow as every 320 ms.
There are two basic problems with the existing techniques. First, the mobile speed or Doppler alone is not the only factor which leads to the predictability of the channel state. Proper rate selection by the base station scheduler requires an accurate estimate of the signal to noise plus interference ratio (SINR). While mobile speed or Doppler is indicative of the predictability of the signal power, it does not give any indication as to the variability in the interference. Due to the narrowband scheduling capability of the LTE uplink, the variability in interference generated by mobiles in other cells can potentially be quite large, and this is not predictable by the base station scheduler, and hence choosing an HARQ operating point based on low mobile speed or Doppler alone would not be appropriate.
Second, the SRS period that is configured will depend on the number of active users which are connected; this is due to the fact that only 16 users at most can transmit in one SRS transmission opportunity according to the 3GPP LTE specification. Therefore, when a very large number of active users are present, the SRS period must be increased in order to accommodate the larger user count, and further, different users may be configured with different SRS periods. This means an appropriate HARQ operating point must be selected for each of the possible SRS periods and possible mobile speeds. As the 3GPP specification matures and different SRS periods are added, the base station software must be updated to include the appropriate HARQ operating point for this SRS period.
Thus, new mechanisms and techniques that are able to address some of the current issues in establishing and adapting an HARQ operating point would advance communications generally.
Specific embodiments of the present invention are disclosed below with reference to FIGS. 1-6. Both the description and the illustrations have been drafted with the intent to enhance understanding. For example, the dimensions of some of the figure elements may be exaggerated relative to other elements, and well-known elements that are beneficial or even necessary to a commercially successful implementation may not be depicted so that a less obstructed and a more clear presentation of embodiments may be achieved. In addition, although the logic flow diagrams above are described and shown with reference to specific steps performed in a specific order, some of these steps may be omitted or some of these steps may be combined, sub-divided, or reordered without departing from the scope of the claims. Thus, unless specifically indicated, the order and grouping of steps is not a limitation of other embodiments that may lie within the scope of the claims.
Simplicity and clarity in both illustration and description are sought to effectively enable a person of skill in the art to make, use, and best practice the present invention in view of what is already known in the art. One of skill in the art will appreciate that various modifications and changes may be made to the specific embodiments described below without departing from the spirit and scope of the present invention. Thus, the specification and drawings are to be regarded as illustrative and exemplary rather than restrictive or all-encompassing, and all such modifications to the specific embodiments described below are intended to be included within the scope of the present invention.